Internal combustion engines typically employ mechanical, electrical or hydro-mechanical valve actuation systems to control the flow of combustible components, typically fuel and air, to one or more combustion chambers during operation. Such systems control the motion and timing of intake and exhaust valves during engine operation and may include a combination of camshafts, cam followers, rocker arms, push rods and other elements (such elements, in combination, constituting a valve train), which are driven by a rotating engine crankshaft. The timing of valve actuation may be fixed by the size and location of the lobes on the camshaft.
During positive power operation, for each full 360-degree rotation of the camshaft, the engine sequentially completes an intake stroke, compression stroke, power or expansion stroke and then an exhaust stroke. During the intake stroke, intake valves are opened to admit fuel and air into a cylinder for combustion. During the compression stroke, both exhaust and intake valves are closed to permit compression by a piston of the air fuel mixture in the combustion chamber. The exhaust and intake valves remain closed as the compressed air/fuel mixture explodes forcing the piston downward in the expansion or power stroke. During the exhaust stroke, exhaust valves are subsequently opened to allow combustion products to escape the cylinder. Valve motion during this four-stroke operation is typically referred to as “main event” operation of the valves.
In addition to positive power main event operation, valve actuation systems may be configured to facilitate “auxiliary events” during auxiliary engine operation. For example, it may be desirable to actuate (lift) the exhaust valves during a compression stroke for compression-release (CR) engine braking, bleeder braking, exhaust gas recirculation (EGR), brake gas recirculation (BGR) or other auxiliary valve events. Other auxiliary valve actuations applied to intake valves are also known in the art. During these auxiliary events, valve timing and motion may be controlled in a different manner compared to the main event operation.
For auxiliary events, “lost motion” devices may be utilized in the valve train to facilitate auxiliary event valve movement. Lost motion devices refer to a class of technical solutions in which valve motion is modified compared to the motion that would otherwise occur as a result of a respective cam surface alone. Lost motion devices may include devices whose length, rigidity or compressibility is varied and controlled in order to facilitate the occurrence of auxiliary events in addition to main event operation of valves.
So-called 4-stroke compression release engine braking, which augments main event valve motion by providing energy dissipation events via controlled exhaust valve lift during each compression stroke—corresponding to every other instance of piston top dead center (TDC)—has long been known. More recent developments in engine braking include enhanced engine braking systems, such as those marketed under the names HIGH POWER DENSITY™ and HPD™ by Jacobs Vehicle Systems, Inc. of Bloomfield, Conn. Examples of such systems and methods are described in U.S. Pat. No. 8,936,006, the subject matter of which is incorporated herein in its entirety. These engine braking systems provide increased energy dissipation, compared to 4-stroke compression release engine braking, by utilizing valve motions that result in energy dissipation events corresponding to every instance of TDC. In these braking systems, a braking “2-stroke” implementation may cancel the main event motions on the intake and exhaust valves using lost motion devices and may add secondary or auxiliary motions to one or more of the intake and/or exhaust valves such that compression release events correspond to each instance of TDC. A variation of “2-stroke” braking is “1.5-stroke” braking in which the main event motion on the exhaust valve is cancelled while adding a secondary braking motion on one more of the exhaust valves. In “1.5-stroke” braking, the intake valve main event motion remains unchanged. Such systems may include a “failsafe” feature in which low lift of the exhaust valve is provided to prevent a completely sealed condition of the cylinder.
State-of-the-art engine braking systems require more precise and complex valve motion deactivation of such braking systems, otherwise loads exceeding steady state may occur on the intake valve train and last for one or more engine cycles. This excessive loading may arise, for example, when the main event exhaust motion has been “lost” or cancelled before a hydraulically actuated braking piston associated with an exhaust valve has been allowed sufficient time to index to its steady state position. This may result in occurrence of the low lift “failsafe” event from the braking rocker to reduce transient intake cylinder pressure. However, such failsafe systems still cannot evacuate as much air from the cylinder as a main event exhaust lift can. As a result, the intake rocker may open one or more valves against higher than normal cylinder pressure, resulting in high load to the intake rocker and valve train, as well as leaving the intake manifold and flow path exposed to a high-pressure pulse, which may lead to undesirable consequences, such as counter flow in the intake manifold and surge of an upstream turbocharger in the intake flow path.
It would therefore be advantageous to provide systems and methods that address the aforementioned shortcoming and others in the prior art.